I. Field of the Invention
This invention relates generally to digital communication systems and more particularly to terminal units for collecting and transmitting data over utility power transmission lines.
II. Description of the Prior Art
Electronic communication systems commonly make use of a carrier frequency which is modulated by the information to be transmitted. For example, information may be transmitted over power lines between a central facility and a multiplicity of remote terminal units, each tied to the power distribution network. Power line communication systems of this type are disclosed and claimed in U.S. Pat. No. 3,973,240, issued Aug. 3, 1976 to Fong; U.S. Pat. No. 3,973,087, issued Aug. 3, 1976 to Fong; U.S. Pat. No. 3,944,723, issued Mar. 16, 1976 to Fong; U.S. Pat. No. 4,135,181, issued Jan. 16, 1979 to Bogacki, et al; U.S. Pat. No. 4,161,720, issued July 17, 1979 to Bogacki; and U.S. Pat. No. 4,315,251, issued Feb. 9, 1982 to Robinson, et al.; all assigned to the assignee of the present application and all incorporated herein by reference in their entirety.
In most power line communication systems, data is gathered and transmitted by remote terminal units to a central receiving and control unit, in response to commands sent by the central unit to the remote unit over the utility power lines. These utility power lines are characterized by networks having many branches, each branch having its own frequency attenuation characteristics which may be quite different from other branches. This implies that the optimum frequency for use on one branch, will not be optimum on some other branch of the system. This also implies that a given branch will work better at one frequency than at another. It has been found that difference in propagation within the frequency band of 5 khz. to 10 khz. can be 20 db or more. In other words, there is usually one best frequency for each remote terminal unit location, and there can be a 20 db. difference between that and the worst frequency, in the 5 to 10 khz. band. Consequently, it can be seen that the use of a single frequency for transmitting data from the remote units to the central receiving and control unit can lead to problems in reliably acquiring information from the remote units.
To overcome the aforementioned transmission difficulties, prior systems have used various frequency assignment plans. Some have used a single compromise frequency for all locations, which may work well at some locations but poorly at others. In other systems, data is encoded in a frequency pattern, requiring that all frequencies work well at a location for that location to operate. Still another system utilizes a frequency multiplexed system wherein a group of remote units, each remote unit within the group having a different assigned frequency, simultaneousls transmit data to the central unit (see the aforementioned U.S. Pat. No. 4,315,251. Although, this system is an improvement over the prior single frequency systems, one must measure the power line frequency attenuation characteristics at each terminal unit location, in order to find the best one of the available frequencies assigned to the group to use at that location, then dedicate that single frequency to that particular remote unit. As can be readily seen, this type of frequency optimization would add costs and complexity to the installation procedure of the remote units as well as requiring a selection of units by frequency at each particular installation. In addition, even after the optimization procedure has been carried out, the frequency attenuation characteristics at a particular site can still change due to the capability possessed by most power companies of switching branches in and out for redistribution of loads within a network; and switching capacitor banks in and out for load factor adjustment. Consequently, even though a particular remote unit is installed on a particular branch, and has been assigned the optimum frequency for that branch at the time of installation, the propagation versus frequency characteristics of that particular branch might still change.
Another problem relating to the reliable transmission of data over power lines is the noise which appears on those lines at random times. Unlike the propagation versus frequency characteristics of the power lines which are uniquely associated with the particular physical and electrical configuration of a transmission line, noise is usually a time related random variable caused by the operation of "noisy" equipment, such as electric motors which are connected to the power distribution network. The noise appears on the network only when the equipment is operated and this operation can take place at any time. This noise can prevent the reliable reception of information transmitted over the power network at either a particular frequency (narrow-band noise) or at many different frequencies simultaneously (broad-band noise).